readertest.cpp: use top-level Parse functions, instead of internal ones
readertest.cpp: use top-level Parse functions, instead of internal ones

With the allowance of arbitrary root value types, the individual tests
can use the top-level Parse functions, instead of ParseFoo() variants.

Secondly, some unneeded array wrappers have been dropped and non-singular
tests starting with other values than objects or arrays have been added.

zhengxle/rapidjson,zhengxle/rapidjson,zhengxle/rapidjson